Cells increase in number by dividing in two. Each daughter cell receives half the contents of the mother cell and an identical copy of the chromosomes - the genetic instructions. During preparation for division, the chromosome pairs are collected on a spindle shaped microtubule scaffold. The pairs are then separated to the two poles of the spindle. Subsequently the poles move further apart, taking the chromosomes with them and the cell membrane constricts into the spindle midplane, pinching the cell into two daughters each with an exact copy of the genetic material and half of the cytoplasm of the original. This description is a greatly simplified version of a very complicated and tightly regulated process. The microtubules are the central players;they define the basic spindle architecture, they are the tracks that guide chromosomes to their destinations at the various stages and they are the substrates that molecular motors use to push the spindle poles apart before the cytoplasm is divided. The process is of fundamental importance for both healthy and diseased cells. As cancer cells are dividing uncontrollably, the spindle is a common target for anti-cancer drugs. The proposed work is a series of structure determinations aimed at visualizing how critically important proteins and macromolecular complexes interact at foci of activity on the microtubules. The regions of interest are the plus and minus ends of microtubules. Here coordinated microtubule assembly and disassembly result in chromosome collection followed by separation of the chromosome pairs. Of particular interest are proteins responsible for chromosome attachment to disassembling microtubule plus ends, proteins that bind and track with the plus ends and a protein that caps and protects minus ends. The other region of interest is the zone in the center of the spindle where microtubules from opposite poles overlap. Here crosslinking proteins and motors determine and regulate spindle length. The structure determinations will be carried out by cryo-electron microscopy and image analysis. The structural results we obtain will provide the basis for a mechanistic understanding of some of the most important events taking place on spindle microtubules during cell division. PUBLIC HEALTH RELEVANCE: The proposed research is aimed at understanding fundamental processes that operate when cells divide. As cancer cells are dividing in an uncontrolled fashion, understanding these processes may provide the basis for developing anti-cancer drugs.